JP2005-3193A describes a known technique to control a vehicle provided with a torque converter including a lock-up clutch so as to improve the torque transmission efficiency and fuel efficiency by performing slip control for the lock-up clutch when the vehicle starts.
When a progressive load of the vehicle is large, e.g., when a live load of the vehicle is large, while towing, and when starting on a climbing road, the lock-up clutch is subjected to an excessive load if the slip control for the lock-up clutch is performed whenever the vehicle starts. This gives rise to the possibility of deterioration in the driving performance because a torque capacity of the lock-up clutch becomes overabundant, an engine speed does not rapidly increase, and a sufficient driving force cannot be obtained. Also, the excessive load could possibly lower the durability of the lock-up clutch.
Such troubles may be avoided by starting in a converter state, in which the lock-up clutch is disengaged, when the vehicle is subjected to a large load. However, this gives rise to the problem that the engine speed rapidly increases and the fuel efficiency decreases when, for example, an accelerator pedal opening operated by a driver is large.